Until recently, poly(alkylene carbonates) have had limited commercial application. They have been used as sacrifice polymers in the electronics industry but in few other applications. Other applications of these polymers have been limited, among others, by their relative thermal instability.
The present inventors have realised that these poly(alkylene carbonates) offer environmentally friendly potential. The use of carbon dioxide in the formation of poly(alkylene carbonates) provides a useful sink for carbon dioxide and therefore these polymers offer an environmentally friendly alternative to fossil fuel based materials such as a polyolefin. There are therefore significant benefits to using PACs industrially.
As noted above, commercial applications of poly(alkylene carbonates) are limited by their low thermal stability. Furthermore, thermal decomposition of these polymers occurs at rather low temperatures, e.g. at 180° C. for poly(propylene carbonate) (PPC). These properties severely limit the processability of PACs on a commercial scale. Methods of broadening the properties and processing window so as to enhance the applicability of PACs are therefore sought. Crosslinking the PAC is one route which has been investigated and, to date, some progress has been achieved.
There remains, however, a need for new methods to produce significant quantities of PAC's, which possess advantageous properties such as improved thermal stability. In particular, the inventors sought new processes which produce PAC's with higher glass transition temperatures and/or enhanced higher temperature thermal stability over those known in the art are needed.
The present inventors have surprisingly established that careful control of polymer purification procedures can give rise to polymers with improved thermal properties. The resulting materials exhibit beneficial properties, in particular in terms of rigidity and thermal stability.
Moreover, the inventors have found that purification can be successfully effected in the absence of organic solvents. In typical purification procedures for aliphatic polycarbonates, the use of organic solvents is normal. The polymer formed contains catalyst residues and in order to remove these, organic solvents are routinely used, in particular chlorinated solvents such as chloroform and dichloromethane. These solvents are potentially toxic but are definitively expensive. Moreover, the quantities of solvent employed can be vast. Even if the solvent is recycled, there are still expensive separation and purification procedures needed to reuse solvent.
Organic solvents are also believed essential to remove certain impurities from the formed polycarbonate. For example, the polymerisation of propylene oxide and carbon dioxide forms polypropylene carbonate and may also form propylene carbonate. This compound is believed to cause melt fracture, act as a plasticizer and make the resin tacky. To remove this impurity, the use of dichloromethane and methanol or other solvents and antisolvents is conventional. Typically, the process involves a liquid-liquid extraction. After dissolution in the solvent, the polycarbonate material is washed with an aqueous acid and subsequently a non solvent such as methanol is added to precipitate the polycarbonate.
These organic solvents also dissolve oligomeric portions of the polycarbonate however. They also remove portions of the PAC in which there is a high ether linkage content. Whilst removing some of these oligomeric compounds might be regarded as advantageous in order to reduce migration, the amounts lost in organic solvent may be significant. Valuable PAC is simply being removed with the organic solvent.
Peng, Polymer degradation and stability 80 (2003) 141-147, suggests end capping as a means for improving polymer stability by decreasing the chain unzipping from the hydroxyl end groups in the PAC at low temperatures Peng achieves end capping in organic solvent. In a similar disclosure Yao et al, J Appl Polm Sci vol 120, 3565-3573 (2011) achieves that result by melt blending with maleic anhydride. Further more, thermal stability can be further improved by the addition of calcium stearates and stearic acid according to Yu et al in J Appl Polm Sci vol 120, 690-700 (2011).
There are therefore various methods available to improve thermal stability. However, the amount of acid applied, in particular to cause end capping, is high. Resins from the prior art with reported enhancement of thermal stability, suffer from the presence of acid residues that may represent a problem for further processing (corrosion of machinery) and application (migration of acid residues).
In the present invention, the amount of acid residues present will be lower since the products have been subjected to a thorough washing as part of the treatment. Additionally, the processes used in the prior art may be insufficient in the removal of unwanted side products and may likewise leave undesirable high solvent residues in the polymer. Also, the prior art precipitation method from solvent mixtures may not give polymer particles suitable for further handling as agglomeration is an issue.